The ultrastructure of spermatozoa and spermiogenesis inPolydora neocaecais described. Most morphological features and measurements of cellular components of the spermatozoa are similar to previously studied members of the genusPolydora.However, early spermatids contain finely granular chromatin which becomes finely fibrillar in late spermatids and uniformly electron-dense in mature spermatozoa. Fibrillar chromatin morphology has not been reported within the Spionidae and may represent a species-specific trait or a previously unrecognized transient developmental process. Spermatozoa ofPolydora neoceacaexhibit an elongate, cylindrical nucleus and a bullet-shaped acrosome that contains channels of electron transparent material. No microtubules are associated with nuclear elongation. The basal body of the axoneme occupies a short implantation fossa in the nucleus. The middlepiece contains 6–10 mitochondria closely aligned with the axoneme. Platelets, presumably carbohydrate storage bodies, are found along the nucleus and middlepiece. Additional electron-dense granules occur in the cytoplasm of the middlepiece.